Electron tube with electron-bombarded semiconductor device

ABSTRACT

A photocathode is formed on a predetermined portion of the internal surface of an envelope of an electric tube. An avalanche photodiode (APD) is provided inside the envelope. The APD is surrounded by a cover and a tubular inner wall. A manganese bead and an antimony bead serving as evaporation sources are disposed in the vicinity outside the inner wall. The manganese bead and the antimony bead are surrounded by a tubular outer wall. The manganese bead and the antimony bead generate metal vapor to thereby form the photocathode. In forming the photocathode, the cover, inner wall, outer wall prevent the metal vapor from being deposited on the APD or an unintended portion inside the electron tube.

TECHNICAL FIELD

The present invention relates to an electron tube.

BACKGROUND ART

Various electron tubes have been proposed. The electron tubes have aphotocathode that emits photoelectrons in response to an incident lightand a detection section constituted by a semiconductor device or amultiple-stage dynode that amplifies the photoelectrons so as to detectthem.

As an electron tube using the multiple-stage dynode, there is availablean electron tube in which a photoelectron emission photocathode isformed on a faceplate provided at the end portion of a tubular envelopeand a multiple-stage dynode is provided opposed to the faceplate. Thiselectron tube has, on the faceplate, an evaporator for depositing amaterial for use in formation of the photoelectron emission cathode. Theevaporator is provided outside a tube surrounding the dynodes andprevents the material evaporated from the evaporator from being adheredto the dynodes. Further, a plurality of focusing electrodes is providedin the electron tube. These electrodes prevent the material evaporatedfrom the evaporator from being adhered to an unintended portion, such asthe internal wall of the envelope (refer to, for example, PatentDocument 1).

As an electron tube using the semiconductor device, there is availablean electron tube that encapsulates therein an electron-irradiated typediode. In this electron tube, a shied plate that restricts the electronpath is provided around the semiconductor device (refer to, for example,Patent Document 2).

As an electron tube using an avalanche photodiode (hereinafter, referredto as APD) as the semiconductor device, there has been proposed anelectron tube in which an entrance window and a conductive stem aredisposed opposite to each other at both ends of an insulating container;a photocathode is formed on the internal wall of the entrance window;and the APD is disposed on the conductive stem. The conductive stemprotrudes in the direction toward the photocathode. In forming thephotocathode on the entrance window, metal vapor such as alkali metalvapor are injected through a through-hole formed in the insulatingcontainer, in a predetermined order to allow the metal vapor to reactwith previously deposited antimony (refer to Patent Document 3).

[Patent Document 1]

Japanese Patent Application Laid-Open Publication No. 2-288145 (pages 3to 4)

[Patent Document 2]

Japanese Patent Application Laid-Open Publication No. 6-318447 (pages 5to 8, FIG. 1)

[Patent Document 3]

Japanese Patent Application Laid-Open Publication No. 9-297055 (pages 4to 9, FIG. 4)

DISCLOSURE OF INVENTION Objects of the Invention

When the semiconductor device and dynodes are used as the electrondetection section, the semiconductor device is excellent in responsespeed, leak current characteristic, and cost performance, relative tothe dynodes.

An object of the present invention is therefore to provide an electrontube having an electron-bombarded semiconductor device and capable ofpreventing metal from being adhered to an undesirable portion with asimple configuration.

ARRANGEMENT SOLVING THE PROBLEM

To attain the above object, the present invention provides an electrontube. The envelope formed with a photocathode at a predetermined part ofthe internal surface thereof; a fixing plate which is disposed in theenvelope and which has a central position and a outer peripherysurrounding the central position; an electron-bombarded semiconductordevice which is fixed to the central position of the fixing plate andwhich faces the photocathode; a first tubular wall which is fixed to aposition between the central position and the outer periphery of thefixing plate, the first tubular wall surrounding the semiconductordevice and extending toward the photocathode; and an evaporation issource generating metal vapor, the evaporation source being disposedinside the envelope on the photocathode side relative to the fixingplate and being disposed at a position between the first tubular walland an imaginary-extended-curved-surface of the outer periphery of thefixing plate that extends toward the photocathode, the semiconductordevice detecting photoelectrons emitted from the photocathode inresponse to an incident light thereon.

According to the above configuration, the photocathode is formed at thepredetermined part of the internal surface of the envelope, the fixingplate is disposed inside the envelope, and semiconductor device and thefirst tubular wall are fixed to the fixing plate. The semiconductordevice is surrounded by the first tubular wall. The evaporation sourceis disposed on the photocathode side relative to the fixing plate in theenvelope and at a position between the first tubular wall and theimaginary-extended-curved-surface of the peripheral of the fixing platethat extends toward the photocathode. The evaporation source generatesmetal vapor to thereby form the photocathode. The semiconductor devicedetects photoelectrons generated from the photocathode.

According to the electron tube having the above configuration, theevaporation source is disposed at a position between the first tubularwall and the imaginary-extended-curved-surface of the peripheral of thefixing plate that extends toward the photodiode. Therefore, the metalvapor can efficiently be deposited on a predetermined area of theenvelope in forming the base film of the photodiode. By limiting thephotodiode to a minimally required area, contribution of a dark current,which is emitted from the portions other than the effective area, to thesignal can be reduced.

Preferably, the electron tube of the present invention further mayinclude an insulating tube having one end and another end, the anotherend being connected to the envelope and the one end protruding insidethe envelope, wherein the fixing plate and the evaporation source aredisposed on the one end of the insulating tube.

According to the above configuration, the fixing plate is disposed onthe one end of the insulating tube. The insulating tube has the anotherend connected to the envelope and the one end protrudes inside theenvelope and faces the photocathode. The semiconductor device isinsulated from the envelope by the insulating tube.

In the electron tube having the above configuration, the semiconductordevice protrudes inside the envelope. Therefore, when a ground voltageand a voltage having a positive polarity are applied to the envelope andsemiconductor device, respectively, a voltage having a high absolutevalue can be prevented from being exposed to the outside environment.Therefore, the electron tube can easily be handled and occurrence ofdischarge between the envelope and outside environment can be prevented.

Preferably, the envelope may include a cylindrical base; and a main bodyhaving a first main body that is curved substantially in a sphericalshape and a second main body that is curved substantially in a sphericalshape and that connects the first main body to the base; and wherein thesemiconductor device is disposed on the main body side relative to anintersection between an axis of the base and an imaginary extendedsurface of the second main body that is located inside the base.

According to the above configuration, the envelope has a base and a mainbody. The base is formed into a cylindrical shape. The main body has thefirst main body and the second main body, which are curved substantiallyin a spherical shape. The second main body connects the first main bodyand the base. The semiconductor device is disposed on the main body siderelative to an intersection between theimaginary-extended-curved-surface of the second main body and thecentral axis of the base.

According to the electron tube having the above configuration, thephotocathode is formed at the predetermined part of the main body whichhas a surface curved substantially in a spherical shape, and thesemiconductor device is disposed on the main body side relative to theintersection between the imaginary-extended-curved-surface of the secondmain body within the base and the central axis of the base. Since beingformed on the surface curved substantially in a spherical shape, thephotocathode can be formed widely. Further, application of a potentialdifference between the photocathode and semiconductor device generatessubstantially a spherical potential gradient around the semiconductordevice. Therefore, the photoelectrons emitted from the photocathodehaving a wide effective area can be converged on the semiconductordevice having a small effective area. Thus, the generated electrons areconverged on the semiconductor device and enter the semiconductor deviceefficiently, thereby increasing electron detection sensitivity. Further,since the size of the semiconductor device itself is small, the electrontube according to the present invention has high-speed response, smallleak current, and can be protruded at a low manufacturing cost.

Preferably, the another end of the tube may be connected to the envelopeand the one end of the tube protrudes inside the main body of theenvelope, and wherein the fixing plate and the evaporation source aredisposed on the one end of the tube.

According to the above configuration, the one end of the insulating tubeprotrudes inside of the envelope. The another end is connected to theenvelope. The fixing plate and the evaporation source are disposed onthe one end of the tube.

In the electron tube having the above configuration, the semiconductordevice protrudes inside of the envelope. Therefore, when a groundvoltage and a voltage having a positive polarity are applied to theenvelope and semiconductor device, respectively, a voltage having a highabsolute value can be prevented from being exposed to the outsideenvironment. Therefore, the electron tube can easily be handled andoccurrence of discharge between the envelope and outside environment canbe prevented.

Preferably, the electron tube of the present invention may includefurther a conductive member provided on the one end of the tube andprotruding outside the tube to reduce the field intensity in thevicinity of the one end of the tube, wherein the fixing plate includesan inner stem that is connected to the one end of the tube via aconductive member.

According to the above configuration, the inner stem is connected to theone end of the insulating tube via the conductive member, and thesemiconductor device is provided on the inner stem. Further, theconductive member is formed protruding from the one end of theinsulating tube. The conductive member reduces the field intensity inthe vicinity of the one end of the insulating tube.

According to the electron tube having the above configuration, the fieldintensity in the one end of the insulating tube is reduced by theconductive member, thereby preventing occurrence of discharge.Therefore, a large potential difference can be applied between thephotocathode and semiconductor device to thereby increase detectionefficiency.

Preferably, the electron tube of the present invention further mayinclude a conductive member provided on the another end of the tube andprotruding outside the tube to reduce the field intensity in thevicinity of the another end of the tube, wherein the envelope includesan outer stem connected to the another end of the tube, at least a partof the outer stem that is connected to the another end of the tube beingconductive.

According to the above configuration, the envelope has the outer stem.The outer stem is connected to another end of the tube. At least a partof the outer stem that is connected to the another end of the tube isconductive. Further, the conductive member is provided protruding fromthe another end of the insulating tube. The conductive member reducesthe field intensity in the vicinity of the another end of the tube.

According to the electron tube having the above configuration, the fieldintensity in the another end of the insulating tube is reduced by theconductive member, thereby preventing occurrence of discharge.Therefore, a large potential difference can be applied between thephotocathode and semiconductor device to thereby increase detectionefficiency.

Accordingly another aspect, the invention provides an electron tubeincluding an envelope formed with a photocathode in a predetermined partof an internal surface thereof; an electron-bombarded semiconductordevice provided inside the envelope; a first tubular wall whichsurrounds the semiconductor device; an evaporation source that generatesmetal vapor, the evaporation source being disposed within the envelopeand outside the first tubular wall; and a second tubular wall whichsurrounds the evaporation source, the semiconductor device detectingphotoelectrons emitted from the photocathode in response to an incidentlight thereon.

According to the above configuration, the photocathode is formed at thepredetermined part of the internal surface thereof. The semiconductordevice is provided inside the envelope and is surrounded by the firsttubular wall. The evaporation source is disposed outside the firsttubular wall. The evaporation source is surrounded by the second tubularwall. The evaporation source generates metal vapor to thereby form thephotocathode. The semiconductor device detects photoelectrons generatedfrom the photocathode.

According to the electron tube having the above configuration, theevaporation sources are surrounded by the second tubular wall.Therefore, at the time when the photocathode is formed, a simplestructure, i.e., the tubular wall can prevent the metal vapor from beingadhered to a portion other than the predetermined area of the envelope.By limiting the photocathode to a minimally required area, contributionof a dark current, which is emitted from the portions other than theeffective area, to the signal can be reduced.

Preferably, the electron tube of the present invention may includefurther an insulating tube having one end and another end, the anotherend being connected to the envelope and the one end protruding insidethe envelope, wherein the semiconductor device, the first tubular wall,the evaporation source, and the second tubular wall are disposed on theone end of the tube.

According to the above configuration, the semiconductor devicesurrounded by the first tubular wall and evaporation sources surroundedby the second tubular wall are provided at the one end of the insulatingtube. The insulating tube has the one end and the another end. Theanother end is connected to the envelope and the one end protrudesinside of the envelope. The semiconductor device is insulated from theenvelope by the insulating tube.

In the electron tube having the above configuration, the semiconductordevice protrudes inside the envelope. Therefore, when a ground voltageand a voltage having a positive polarity are applied to the envelope andsemiconductor device, respectively, a voltage having a high absolutevalue can be prevented from being exposed to the outside environment.Therefore, the electron tube can easily be handled and occurrence ofdischarge between the envelope and outside environment can be prevented.

Preferably, the envelope may include a cylindrical base; and a main bodyhaving a first main body that is curved substantially in a sphericalshape and a second main body that is curved substantially in a sphericalshape and that connects the first main body to the base; and wherein thesemiconductor device is disposed on the main body side relative to anintersection between an axis of the base and animaginary-extended-curved-surface of the second main body that islocated inside the base.

According to the above configuration, the envelope has a base and a mainbody. The base has a tubular shape. The main body includes a first mainbody and a second main body which are curved in a spherical shape. Thesecond main body connects the first main body and the base. Thesemiconductor device is disposed on the main body side relative to anintersection between an imaginary-extended-curved-surface of the secondmain body and the central axis of the base.

According to the electron tube having the above configuration, thephotocathode is formed at a predetermined part of the main body having asurface curved in a spherical shape, and the semiconductor device isdisposed on the main body side relative to an intersection between theimaginary-extended-curved-surface of the second main body within thebase and the central axis of the base. Since being formed on the surfacecurved in a spherical shape, the photocathode can be formed widely.Further, application of a potential difference between the photocathodeand semiconductor device generates substantially a spherical potentialgradient around the semiconductor device. Therefore, the photoelectronsemitted from the photocathode having a wide effective area can beconverged on the semiconductor device having a small effective area.Thus, the generated electrons are converged on the semiconductor deviceand enter the semiconductor device efficiently, thereby is increasingelectron detection sensitivity. Further, since the size of thesemiconductor device itself is small, the electron tube according to thepresent invention has high-speed response and small leak current. Thusmanufacturing of the electron tube is easy. Since the manufacturing ofthe electron tube becomes easier, manufacturing cost thereof is reduced.

Preferably, the another end of the tube may be connected to the envelopeand the one end of the tube protrudes inside the main body of theenvelope, and wherein the semiconductor device is disposed on the oneend of the tube.

According to the above configuration, the one end of the insulating tubeprotrudes inside of the main body of the envelope. The another end ofthe tube is connected to the envelope. The semiconductor device isprovided at the one end of the tube.

In the electron tube having the above configuration, the semiconductordevice protrudes inside the envelope. Therefore, when a ground voltageand a voltage having a positive polarity are applied to the envelope andsemiconductor device, respectively, a voltage having a high absolutevalue can be prevented from being exposed to the outside environment.Therefore, the electron tube can easily be handled and occurrence ofdischarge between the envelope and outside environment can be prevented.

Preferably, the electron tube of the present invention may furtherinclude: an inner stem connected to the one end of the tube via aconductive member; and a conductive member provided on the one end ofthe tube and protruding outside the tube to reduce the field intensityin the vicinity of the one end of the tube, wherein the semiconductordevice is disposed on the inner stem.

According to the above configuration, the inner stem is connected to theone end of the insulating tube via the conductive member, and thesemiconductor device is provided on the inner stem. Further, theconductive member is formed protruding from the one end of theinsulating tube and protrudes. The conductive member reduces the fieldintensity in the vicinity of the one end of the insulating tube.

According to the electron tube having the above configuration, the fieldintensity in the one end of the insulating tube is reduced by theconductive member, thereby preventing occurrence of discharge.Therefore, a large potential difference can be applied between thephotocathode and semiconductor device to thereby increase detectionefficiency.

Preferably the electron tube of the present invention may includefurther a conductive member provided on the another end of the tube andprotruding outside the tube to reduce the field intensity in thevicinity of the another end of the tube, wherein the envelope includesan outer stem connected to the another end of the tube, at least a partof the outer stem that is connected to the another end of the tube beingconductive.

According to the above configuration, the envelope has the outer stem.The outer stem is connected to the another end of the tube. At least apart of the outer stem that is connected to the another end of the tubeis conductive. Further, the conductive member is provided protrudingfrom the another end of the insulating tube. The conductive memberreduces the field intensity in the vicinity of the another end of theinsulating tube.

According to the electron tube having the above configuration, the fieldintensity in the another end of the insulating tube is reduced by theconductive member, thereby preventing occurrence of discharge.Therefore, a large potential difference can be applied between thephotocathode and semiconductor device to thereby increase detectionefficiency.

Preferably, the envelope may be applied with a ground potential, and thesemiconductor device is applied with a positive potential.

According to the above configuration, a ground potential is applied tothe envelope and a positive potential is applied to the semiconductordevice. The envelope is electrically insulated from the semiconductordevice by the insulating tube.

In the electron tube having the above configuration, a voltage having apositive polarity is applied to the semiconductor device protrudinginside the envelope and a ground voltage is applied to the envelopeexposed to the outside, preventing a voltage having a high absolutevalue from being exposed to the outside environment. As a result, theelectron tube can easily be handled and occurrence of discharge betweenthe envelope and outside environment can be prevented. Therefore, theelectron tube can be used for single photon detection in water, such asthe water Cerenkov experiment or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an electron tubeaccording to an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view taken along the line II-II inthe electron tube of FIG. 1.

FIG. 3 is a vertical cross-sectional view of an electron detectionsection provided in the electron tube of FIG. 1 illustrating anelectrical circuit provided in the electron detection section in detail.

FIG. 4 is a plan view showing an electron detection section head portionas viewed from above.

FIG. 5 is a cross-sectional view schematically showing an APD in theelectron detection section.

FIG. 6 is a perspective view schematically showing the electrondetection section head portion when a shield portion is not provided.

FIG. 7 is a perspective view schematically showing the electrondetection section head portion.

FIG. 8 (A) and FIG. 8 (B) are views showing an alkali source, whereinFIG. 8 (A) is a front view of the alkali source, and FIG. 8 (B) is aschematic perspective view of the alkali source.

FIG. 9 is a vertical cross-sectional view schematically showingequipotential surfaces E and electron trajectories L in the electrontube.

FIG. 10 is a vertical cross-sectional view schematically showingequipotential surfaces E and electron trajectories L in an electron tubeof a comparative example.

FIG. 11 is a vertical cross-sectional view schematically showingequipotential surfaces E generated in the vicinity of upper and lowerend portions of an insulating tube 9 by conductive flanges 21 and 23.

FIG. 12 is a vertical cross-sectional view schematically showingequipotential surfaces E generated in the vicinity of upper and lowerend portions of an insulating tube 9 when the conductive flange 21 or 23is not provided.

FIG. 13 is a vertical cross-sectional view schematically showingequipotential surfaces E and electron trajectories L in the case wherethe vertical cross-section of a glass bulb body is formed into acircular shape.

FIG. 14 is a vertical cross-sectional view schematically showingequipotential surfaces E and electron trajectories L in a comparativeexample.

FIG. 15 is a vertical cross-sectional view showing the outer peripheryof the conductive flange according to a modification.

FIG. 16 is a vertical cross-sectional view showing the configuration ofa shield portion according to another modification.

FIG. 17 is a vertical cross-sectional view showing the configuration ofthe shield portion according to still another modification.

EXPLANATION OF REFERENCE NUMBERS

-   1: Electron tube-   2: Envelope-   3: Glass bulb-   4: Glass bulb body-   4 a: Upper hemisphere-   4 b: Lower hemisphere-   5: Glass bulb base-   6: Outer stem-   9: Insulating tube-   10: Electron detection section-   15: APD-   21, 23: Conductive flange-   26: Partition wall-   27: Alkali source-   60: Stem bottom-   61: Stem inner wall-   62: Stem outer wall-   70: Shield portion-   71: Cover-   72: Inner wall-   73: Cap-   74: Outer wall-   80: Inner stem-   87: Base-   89: Conductive support portion-   90: Electrical circuit-   I: Imaginary extended curved surface of lower hemisphere 4 b-   M: Imaginary extended curved surface of outer periphery 87 b-   S: Reference point-   Z: Axis

BEST MODE FOR CARRYING OUT THE INVENTION

An electron tube according to an embodiment of the present inventionwill be described below with reference to FIGS. 1 to 17.

FIG. 1 is a vertical cross-sectional view schematically showing anelectron tube 1 according to the embodiment of the present invention.

As shown in FIG. 1, the electron tube 1 includes an envelope 2 and anelectron detection section 10. The envelope 2 has an axis Z. Theelectron detection section 10 protrudes inside the envelope 2 along theaxis Z. The electron detection section 10 has substantially acylindrical shape extending with its central axis being located on theaxis Z.

The envelope 2 has a glass bulb 3 and an outer stem 6. The glass bulb 3is formed from a transparent glass.

The glass bulb 3 has a glass bulb body 4 and a cylindrical glass bulbbase 5. The glass bulb body 4 is integrally formed with the glass bulbbase 5. The glass bulb body 4 has substantially a spherical shape havinga central axis located on the axis Z. As shown in FIG. 1, thecross-section of the glass bulb body 4 taken along the axis Z has afirst diameter R1 perpendicular to the axis Z and a second diameter R2parallel to the axis Z. The cross-section of the glass bulb body 4 takenalong the axis Z has substantially an elliptical shape with the firstdiameter R1 longer than the second diameter R2. The cylindrical glassbulb base 5 extends with its central axis being located on the axis Z.

The glass bulb body 4 integrally includes an upper hemisphere 4 a and alower hemisphere 4 b. The upper hemisphere 4 a serves as the upperhemisphere of the glass bulb 4 in the drawing, and is curvedsubstantially spherically to form a semispherical shape. The lowerhemisphere 4 b serves as the lower hemisphere of the glass bulb 4 in thedrawing, and is curved substantially spherically to form a semisphericalshape. Hereinafter, in FIG. 1, the upper hemisphere 4 a is defined asthe upper side with respect to the lower hemisphere 4 a. The lowerhemisphere 4 b is defined as the lower side with respect to the upperhemisphere 4 a. The lower end of the upper hemisphere 4 a is connectedto the upper end of the lower hemisphere 4 b. The lower end of the lowerhemisphere 4 b is connected to the upper end of the glass bulb base 5.The glass bulb 3 is thus integrally formed. A imaginary extended curvedsurface I of the lower hemisphere 4 b crosses the axis Z at a referencepoint S that is located inside the glass bulb base 5.

A photocathode 11 is formed on the internal surface of the upperhemisphere 4 a. The photocathode 11 is a thin film formed by a vapordeposition technique using antimony (Sb), manganese (Mn), potassium (K),and cesium (Cs).

A conductive thin film 13 is formed on the internal surface of the lowerhemisphere 4 b. The upper end of the conductive thin film 13 is broughtinto contact with the lower end of the photocathode 11. Although theconductive thin film 13 is a chromium thin film in this embodiment, thethin film 13 may be formed from an aluminum thin film.

The outer stem 6 is formed from conductive Kovar metal. The outer stem 6includes a stem bottom 60, a stem inner wall 61, and a stem outer wall62. The stem bottom 60 has substantially an annular shape with itscentral axis located on the axis Z and is inclined downward toward theaxis Z. The stem inner wall 61 and stem outer wall 62 have cylindricalshapes with their common central axis coinciding with the axis Z. Thestem inner wall 61 extends upward from the inner edge of the stem bottom60. The stem outer wall 62 extends upward from the outer edge of thestem bottom 60. The upper end of the stem outer wall 62 is air-tightlyconnected to the lower edge of the glass bulb base 5. The upper end ofthe stem inner wall 61 is air-tightly connected to the lower end of theelectron detection section 10. Thus, the electron detection section 10having substantially a cylindrical shape protrudes from the outer stem 6side toward the photocathode 11 side coaxially with the cylindricalglass bulb base 5.

A cylindrical-shaped partition wall 26 is provided between thecylindrical glass bulb base 5 and the substantially cylindrical electrondetection section 10 coaxially therewith. The partition wall 26 isformed, for example, from a conductive material such as a stainlesssteel. The lower end of the partition wall 26 is connected to the stembottom 60. The upper end of the partition wall 26 is located on theupper hemisphere 4 a side (i.e., upper side in FIG. 1) relative to thereference point S with respect to the direction parallel to the axis Z.The upper end of the partition wall 26 is located on the glass bulb base5 side (i.e., lower side) relative to the imaginary extended curvedsurface I of the lower hemisphere 4 b.

Two alkali sources 27, 27 are provided on the outer side surface of thepartition wall 26, i.e., on the side that faces the glass bulb base 5.The two alkali sources 27, 27 are symmetrically provided with respect tothe axis Z. Each of the alkali sources 27, 27 has a support portion 27a, a holding plate 27 b, an attachment portion 27 c, and six containers27 d. In FIG. 1, only two containers 27 d are shown for each alkalisource 27. The containers 27 d are located on the outer stem 6 side(i.e., lower side) relative to the upper end of the partition wall 26with respect to the direction parallel to the axis Z.

An opening 60 a is formed in the stem bottom 60 at the position betweenthe electron detection section 10 and partition wall 26. The opening 60a communicates with an exhaust pipe 7. The exhaust pipe 7 is formed, forexample, from Kovar metal.

A glass tube 63 is connected to the exhaust pipe 7. The glass tube 63 isformed from, for example, Kovar glass. The glass tube 63 is sealed at anend portion 65 thereof.

The electron detection section 10 has an insulating tube 9. Theinsulating tube 9 is formed, for example, from ceramics. The insulatingtube 9 has a cylindrical shape. The insulating tube has a central axisextending along the axis Z.

The lower end of the insulating tube 9 is air-tightly connected to theupper end of the stem inner wall 61. A conductive flange 23 is providedat the lower end of the insulating tube 9. An electron detection sectionhead portion 8 is disposed at the upper end of the insulating tube 9.The electron detection section head portion 8 faces the photocathode 11.A conductive flange 21 is provided at the upper end of the insulatingtube 9. The conductive flanges 21 and 23 protrude in the direction awayfrom the axis Z, i.e., in the direction from the insulating tube 9toward the glass bulb base 5. Each of the conductive flanges 21 and 23has a plate-like shape circumferentially extending on the planeperpendicular to the axis Z. The upper end of the insulating tube 9 islocated on the outer stem 6 side (i.e., lower side) relative to theupper end of the partition wall 26 with respect to the directionparallel to the axis Z.

The electron detection section head portion 8 has a conductive supportportion 89. The conductive support portion 89 has a cylindrical shapewith its central axis being located on the axis Z. The lower end of theconductive support portion 89 is air-tightly connected to the upper endof the insulating tube 9.

The electron detection section head portion 8 further has an inner stem80. The inner stem 80 has substantially a disc shape with its centralaxis being located on the axis Z. The outer edge of the inner stem 80 isair-tightly connected to the upper end of the conductive support portion89. An APD (Avalanche Photodiode) 15, two manganese beads 17, and twoantimony beads 19 are disposed on the inner stem 80. Thus, the innerstem 80 serves as a base plate that holds the APD 15, manganese beads17, and antimony beads 19. Further, on the inner stem 80, a shieldportion 70 for shielding the APD 15, manganese beads 17, and antimonybeads 19 is disposed facing the upper hemisphere 4 a.

The APD 15 is located on the axis Z and on the upper hemisphere 4 a side(i.e., upper side) relative to the reference point S. Further, the APD15 is located on the upper hemisphere 4 a side (i.e., upper side)relative to the upper end of the partition wall 26, with respect to thedirection parallel to the axis Z.

An electrical circuit 90 connected to the electron detection sectionhead portion 8 is encapsulated inside the insulating tube 9 with afilling material 94. The filling material 94 is, for example, aninsulating material such as silicon. The electrical circuit 90 hasoutput terminals N1, N2 and input terminals N3, N4. The output terminalsN1, N2 and input terminals N3, N4 are exposed outside the fillingmaterial 94. The output terminals N1, N2 are connected to an externalcircuit 100. The input terminals N3, N4 are connected to an externalpower supply (not shown).

FIG. 2 is a vertical cross-sectional view taken along the II-II line inFIG. 1. In other words, FIG. 2 shows the vertical cross-section of theelectron tube 1 seeing from the direction different from the directionof the electron tube of FIG. 1 by 90 degrees about the axis Z. In FIG.2, showing of the electrical circuit 90 in the insulating tube 9 isomitted in order to make the overall structure clearer.

Viewed from the angle shown in FIG. 2, a part of the conductive thinfilm 13 extends from the glass bulb body 4 to the glass bulb base 5.This extended part of the conductive thin film 13 is referred to as athin film extension 13 a. A connection electrode 12 extends from thestem bottom 60 and connects the stem bottom 60 with the thin filmextension 13 a. Thus, electrical continuity is established between theconductive thin film 13 and outer stem 6. Accordingly, electricalcontinuity is also established between the photocathode 11 and outerstem 6.

Details of the configuration of the electron detection section 10 willbe described with reference to FIGS. 1 to 7.

FIG. 3 shows the vertical cross-section of the electron detectionsection 10 of FIG. 1 in greater detail. FIG. 4 is a plan view of theelectron detection section head portion 8 of the electron detectionsection 10 as viewed from the photocathode 11 side.

As shown in FIG. 3, the conductive flange 23 is provided at theconnection portion between the insulating tube 9 and conductive steminner wall 61 and is connected to both the insulating tube 9 and steminner wall 61. The conductive flange 23 is formed from a conductivematerial.

The conductive flange 23 has a connection portion 23 a, a flange body 23b, rising portion 23 c, and a rounded leading end 23 d. The connectionportion 23 a has a cylindrical shape and is fixed to the outer surfaceof the cylindrical stem inner wall 61. The flange body 23 b has anannular plate-like shape extending in the direction away from the axisZ. The rising portion 23 c has a cylindrical shape extending upward fromthe outer edge of the flange body 23 b in parallel to the axis Z. Therounded leading end 23 d extends from the upper end of the risingportion 23 c in the direction away from the axis Z. The rounded leadingend 23 d has a greater thickness than those of the connection portion 23a, flange body 23 b, and rising portion 23 c, and has a thick roundedshape.

The conductive flange 21 is provided at the connection portion betweenthe insulating tube 9 and conductive support portion 89 and is connectedto both the insulating tube 9 and conductive support portion 89. Theconductive flange 21 is formed from a conductive material.

The conductive flange 21 has a connection portion 21 a, a flange body 21b, and a rounded leading end 21 c. The connection portion 21 a has acylindrical shape and is fixed to the outer surface of the cylindricalconductive support portion 89. The flange body 21 b has an annularplate-like shape extending in the direction away from the axis Z. Therounded leading end 21 c is formed in the outer circumference of theflange body 21 b. The rounded leading end 21 c has a greater thicknessthan that of the flange body 21 b and has a thick rounded shape.

The conductive support portion 89 is formed from, for example, aconductive material such as Kovar metal.

The inner stem 80 includes an APD stem 16 and a base 87. The base 87 isformed from a conductive material. The base 87 has substantially anannular shape with its center located on the axis Z of the envelope 2.The outer circumference on the lower side surface of the base 87 isfixed to the upper end of the conductive support portion 89. Athrough-hole 87 a is formed in the center of the base 87. Thethrough-hole 87 a has a circular shape with its center located on theaxis Z. The base 87 has an outer periphery 87 b circumferentiallyextending around the axis Z. The outer periphery 87 b defines the outerperiphery of the inner stem 80. As shown in FIGS. 3 and 6, the imaginaryextended curved surface M of the outer periphery 87 b extends from theouter periphery 87 b in the upper direction of FIG. 3 in parallel to theaxis Z. Accordingly, as shown in FIG. 1, the imaginary extended curvedsurface M of the outer periphery 87 b extends from the outer periphery87 b toward the upper hemisphere 4 a (photocathode 11) in parallel tothe axis Z.

The APD stem 16 is fixed to the lower side of the base 87 so as toair-tightly close the through-hole 87 a. The APD stem 16 has a discshape with its center located on the axis Z, and is formed from aconductive material.

The APD 15 is disposed on the APD stem 16 at a position on the axis Zand faces the upper hemisphere 4 a (photocathode 11). Thus, the APD 15is fixed at substantially the center position of the inner stem 80.

Twelve electrodes 83 (FIG. 6) are arranged on the base 87 around thethrough-hole 87 a. Only two electrodes 83 are shown in FIG. 3. Therespective electrodes 83 penetrate the base 87. Each of the electrodes83 is electrically insulated from the base 87 by an insulating material85 such as glass and is air-tightly sealed thereby.

The two manganese beads 17 are symmetrically disposed with respect tothe axis Z. The antimony beads 19 are disposed outside the manganesebeads 17. The two antimony beads 19 are symmetrically disposed withrespect to the axis Z. The manganese beads 17 and antimony beads 19 areheld by wire heaters 81 (see FIGS. 4 and 6), respectively. Each of thewire heaters 81 is connected to corresponding two electrodes 83 (seeFIG. 6) among the twelve electrodes.

As can be seen from FIGS. 1, 3, 4, and 6, the manganese beads 17 andantimony beads 19 are located on the upper side relative to the innerstem 80 (more specifically, the base 87) and disposed on the inner siderelative to the imaginary extended curved surface M of the outerperiphery 87 b of the base 87.

The shield portion 70 is provided to cover the inner stem 80.

As shown in FIGS. 3 and 4, the shield portion 70 includes a cap 73 and acover 71. The cap 73 and cover 71 are formed from conductive material.The cap 73 has a circular cap shape with its central axis located on theaxis Z. The cap 73 has an inner wall 72, an outer wall 74, and a ceiling76 that connects the inner wall 72 and outer wall 74. The inner wall 72and outer wall 74 are of concentric tube shapes with their axis beinglocated on the central axis Z and extend toward the upper hemisphere 4 a(photocathode 11) substantially in parallel to the axis Z, as shown inFIGS. 1 and 3. As shown in FIGS. 1 and 3, the outer wall 74 extends fromthe base 87 substantially along the imaginary extended curved surface Mof the outer periphery 87 b of the base 87 toward the photocathode 11. Athrough-hole 73 a is formed in the center of the ceiling 76. Thethrough-hole 73 a has a circular shape having a central axis located onthe axis Z. Two through-holes 75 are formed in the ceiling 76 atlocations outside the through-hole 73 a. Each of the two through-holes75 has a circular shape. The two through-holes 75 are symmetricallydisposed with respect to the through-hole 73 a. Two through-holes 77 areformed in the ceiling 76 at locations outside the two through-holes 75.Each of the two through-holes 77 has also a circular shape. The twothrough-holes 77 are symmetrically disposed with respect to thethrough-hole 73 a. Each of the manganese beads 17 held by the wireheater 81 is located within the through-hole 75. Each of the antimonybeads 19 held by the wire heater 81 is located within the through-hole77.

The cover 71 is disposed within the through-hole 73 a of the cap 73. Thecover 71 has a circular cap shape having a central axis coinciding withthe axis Z. The cover 71 has an outer wall 71 a and a ceiling 71 b. Theouter wall 71 a has a cylindrical shape having a central axis coincidingwith the axis Z and extends toward the upper hemisphere 4 a(photocathode 11) substantially in parallel to the axis Z, as shown inFIGS. 1 and 3. The outer periphery of the cover 71 (i.e., outer wall 71a) is connected to the inner wall 72 of the cap 73. A through-hole 79 isformed in the ceiling 71 b of the cover 71. The through-hole 79 has acircular shape having a central axis coinciding with the axis Z. Thecover 71 is located above the APD 15.

The cover 71 and inner wall 72 isolate the APD 15 from the manganesebeads 17 and antimony beads 19. The outer wall 74 surrounds themanganese beads 17 and antimony beads 19.

As described above, in the embodiment of the present invention, themanganese beads 17 and antimony beads 19 are disposed at portions on theupper hemisphere 4 a side relative to the base 87 and between theimaginary extended curved surface M of the outer periphery 87 b of thebase 87 and outer wall 71 a of the cover 71. That is, the manganesebeads 17 and antimony beads 19 are disposed at positions that areoutside the outer wall 71 a of the cover 71, and inside the imaginaryextended curved surface M of the outer periphery 87 b of the base 87.That is, the manganese beads 17 and the antimony beads 19 are disposedat positions that are further away from the axis Z than the outer wall71 a. And the manganese beads 17 and the antimony beads 19 are disposedat the positions that are near to the axis Z than the imaginary extendedcurved surface M. Therefore, as described later, the base 87, theceiling 76 of the cap 73, and the outer wall 74 allow the manganesevapor and antimony vapor to be deposited in substantially the entirearea of the internal surface of the upper hemisphere 4 a around the axisZ, while preventing manganese vapor and antimony vapor from beingadhered to the glass bulb base 5, lower hemisphere 4 b, and internalsurface of the outer stem 6. Therefore, a base film of the photocathode11 can be formed in substantially the entire internal surface of theupper hemisphere 4 a. In addition, the cover 71 can prevent themanganese vapor and antimony vapor from being adhered to the APD 15.

A pin 30 is fixed on the lower surface of the APD stem 16. The pin 30 iselectrically connected to the APD stem 16. A pin 32 penetrates the APDstem 16. The pin 32 is electrically insulated from the APD stem 16 andair-tightly sealed by an insulating material 31 such as glass.

The electrical circuit 90 has capacitors C1, C2, an amplifier A1, outputterminals N1, N2, and input terminals N3, N4. The pin 30 and oneterminal of the capacitor C1 are connected to the input terminal N3. Theother terminal of the capacitor C1 is connected to the output terminalN1. The pin 32 and one terminal of the capacitor C2 are connected to theinput terminal N4. The other terminal of the capacitor C2 is connectedto the output terminal N2 through the amplifier A1. The input terminalsN3 and N4 are connected to the external power supply (not shown). Theoutput terminals N1 and N2 are connected to the external circuit 100.The external circuit 100 has a resistor R. The external circuit 100grounds the output terminal N1. The resistor R is connected between theoutput terminals N1 and N2.

Next, the configuration of the APD 15 will be described with referenceto FIG. 5.

As shown in FIG. 5, the APD 15 is disposed on the APD stem 16 so as toface the opening section 79 of the cover 71. The APD 15 is fixed to theAPD stem 16 by a conductive adhesive 49.

The APD 15 has substantially a square plate-shaped n-type highconcentration silicon substrate 41 and a disc-shaped p-type carriermultiplication layer 42 formed on the high concentration siliconsubstrate 41 at substantially the center thereof. A guard ring layer 43is formed around the outer periphery of the carrier multiplication layer42. The guard ring layer 43 has the same thickness as that of thecarrier multiplication layer 42 and is composed of a high concentrationn-type layer. A breakdown voltage control layer 44 composed of a highconcentration p-type layer is formed on the surface of the carriermultiplication layer 42. The surface of the breakdown voltage controllayer 44 is formed as a circular electron incident surface 44 a. Anoxide film 45 and a nitride film 46 are formed so as to extend from theguard ring layer 43 to the area surrounding the breakdown voltagecontrol layer 44.

An incident surface electrode 47 is formed on the outermost surface ofthe APD 15 by depositing aluminum in an annular shape onto the surfacethereof. The incident surface electrode 47 is for supplying thebreakdown voltage control layer 44 with an anode potential. Asurrounding electrode 48 is formed also on the outermost surface of theAPD 15. The surrounding electrode 48 is electrically conducted to theguard ring layer 43. The surrounding electrode 48 is spaced apart fromthe incident surface electrode 47 with a predetermined distance.

The high concentration n-type silicon substrate 41 is electricallyconducted to the APD stem 16 through the conductive adhesive 49.Accordingly, the high concentration n-type silicon substrate 41 iselectrically conducted to the pin 30. The incident surface electrode 47is connected to the penetration pin 32 by a wire 33.

FIG. 6 shows a state where the shield portion 70 has been removed fromthe electron detection section head portion 8 and, further, theconductive flange 21 has been removed from the insulating tube 9 andconductive support portion 89. The conductive support portion 89 isdisposed on the upper portion of the insulating tube 9. The inner stem80 is disposed on the upper portion of the conductive support portion89. The inner stem 80 has the base 87. The APD stem 16 is exposedthrough the through-hole 87 a formed in the base 87.

The APD 15 is disposed on the APD stem 16. The APD 15 has the electronincident surface 44 a that faces upward. The pin 32 is fixed to the APDstem 16. The pin 32 is electrically insulated from the APD stem 16 bythe insulating material 31. The APD 15 is connected to the pin 32 by thewire 33.

The twelve electrodes 83 are fixed to the base 87. Each of theelectrodes 83 is insulated from the base 87 by the insulating material85. The twelve electrodes 83 are circumferentially arranged around thethrough-hole 87 a. Four pairs of electrodes 83 are connected by the wireheaters 81. Each of the wire heaters 81 holds the manganese bead 17 orantimony bead 19. The manganese bead 17 and antimony bead 19 havebead-like shapes.

FIG. 7 shows a state where the conductive flange 21 and shield portion70 have been attached to the electron detection section head portion 8of FIG. 6. The conductive flange 21 is fixed to the upper end of theinsulating tube 9 and is connected to both the insulating tube 9 andconductive support portion 89. The conductive flange 21 extends in thedirection away from the insulating tube 9.

The cap 73 of the shield portion 70 covers the base 87 from above. Thecap 73, which is formed into a circular shape, has the inner wall 72,outer wall 74, and ceiling 76. The circular through-hole 73 a, twothrough-holes 75, and two through-holes 77 are formed in the ceiling 76.The manganese beads 17 held by the wire heaters 81 are exposed throughthrough-holes 75. The antimony beads 19 held by the wire heaters 81 areexposed through through-holes 77. The electron incident surface 44 a ofthe APD 15 is exposed through the through-hole 79 formed on the cover71. The cover 71 and inner wall 72 isolate the APD 15 from the manganesebeads 17 and antimony beads 19. The outer wall 74 surrounds themanganese beads 17 and antimony beads 19.

The configuration of the alkali source 27 will next be described withreference to FIG. 1 and FIGS. 8 (A) and 8 (B). FIG. 8 (A) is a frontview of the alkali source 27 provided outside the partition wall 26 asviewed from the glass bulb base 5 side. FIG. 8 (B) is a perspective viewof the alkali source 27.

The support portion 27 a is formed into an L-like shape having a partextending in parallel to the axis Z and a part extending away from theaxis Z in the radial direction. The support portion 27 a is, forexample, a stainless steel ribbon (SUS ribbon). The part that extends inparallel to the axis Z is fixed to the outer surface of the partitionwall 26.

The holding plate 27 b is fixed to a tip end of a part of a supportportion 27 a that extends in the direction away from the axis Z. Theholding plate 27 b extends in perpendicular to the axis Z andsubstantially in parallel to the circumferential direction of thecylindrical partition wall 26.

The six attachment portions 27 b are fixed to the holding plate 27 b.The containers 27 d are fixed respectively to the tip ends of theattachment portions 27 b. The container 27 d has an opening on its sidesurface. Alkali source pellets (not shown) are contained inside fivecontainers 27 d. A getter (not shown) is contained inside the remainingone container 27 d among the six containers 27 d. The getter is amaterial that absorbs impurity such as barium or titanium.

As shown in FIG. 1, the two alkali sources 27 are disposed in theelectron tube 1. Potassium (K) pellets are contained, as alkali sourcepellets, in five containers 27 d provided in one alkali source 27.Cesium (Cs) pellets are contained, as alkali source pellets, in fivecontainers 27 d provided in the other alkali source 27.

A method of manufacturing the electron tube 1 having the configurationdescribed above will next be described.

Firstly, the glass bulb 3 is prepared by air-tightly connecting the stemouter wall 62 to the lower hemisphere 4 b, with the conductive thin film13 being deposited on the inner surface of the lower hemisphere 4 b.

Further, the stem bottom 60 is prepared with the partition wall 26 andthe connection electrode 12 fixed thereto and with the exhaust pipe 7connected thereto. The two alkali sources 27 and 27 are fixed to thepartition wall 26. The glass tube 63 is connected to the exhaust pipe 7.At this time, the length of the glass tube 63 is larger than that in astate of FIG. 1. Not only the end portion of the glass tube 63 that isconnected to the exhaust pipe 7, but also the opposite end of the glasstube 63 is opened.

Then, the insulating tube 9 is air-tightly connected to the conductivesupport portion 89 of the electron detection section head portion 8. Theconductive flange 21 is connected to the conductive support portion 89and insulating tube 9. The insulating tube 9 is air-tightly connected tothe stem inner wall 61. The conductive flange 23 is connected to theinsulating tube 9 and stem inner wall 61.

Then, the stem inner wall 61 is air-tightly connected to the stem bottom60 by laser welding. The stem outer wall 62 is air-tightly connected tothe stem bottom 60 by plasma welding. As a result, the electron tube 1is obtained with the electron detection section 10 protruding inside theenvelope 2.

Next, the photocathode 11 is formed on the internal surface of the lowerhemisphere 4 a of the glass bulb 3 as described below.

Firstly, an exhaust device (not shown) is connected to the glass tube 63and the inside of the envelope 2 is exhausted through the glass tube 63and exhaust pipe 7. As a result, the inside of the electron tube 1 isset at a predetermined degree of vacuum.

Subsequently, the wire heaters 81 are energized through the electrodes83 to heat the manganese beads 17 and antimony beads 19. To theelectrodes 83, an electrical power is supplied from a power source (notshown). The heated manganese beads 17 and antimony beads 19 generatemetal vapor. The generated vapor of the manganese and antimony isdeposited on the inner surface of the upper hemisphere 4 a to form abase film of the photocathode 11.

At this time, the cover 71, inner wall 72, and outer wall 74 prevent themetal from being deposited on the APD 15 or unintended area of the innersurface of the envelope 2 (to be more specific, the internal surface ofthe lower hemisphere 4 b, glass bulb base 5, or outer stem 6). That is,the cover 71 and inner wall 72 are disposed near the APD 15 so as tosurround the APD 15. Therefore, although the cover 71 and inner wall 72have simple tubular shapes and are small members, they can effectivelyisolate the APD 15 from the manganese beads 17 and antimony beads 19.Therefore, characteristics of the APD 15 can be prevented from beingdegraded due to adhesion of the metal vapor to the APD 15.

The outer wall 74 surrounds the manganese beads 17 and antimony beads19. Therefore, the outer wall 74 can prevent the metal vapor from beingdeposited on the lower hemisphere 4 b, glass bulb base 5, and internalsurface of the outer stem 6.

The manganese beads 17 and antimony beads 19 are disposed, adjacently tothe APD 15, around the APD 15 that is located at substantially thecenter of the inner stem 80. Therefore, the manganese and antimony canbe deposited over a wide area on the internal surface of the upperhemisphere 4 a.

Next, the alkali sources 27, 27 are inductively heated from the outsideof the envelope 2 by electromagnetic induction. Then, the potassium (K)and cesium (Cs) pellets are heated to generate vapor from the openingsof the respective containers 27 d. The potassium and cesium aredeposited on the inner surface of the upper hemisphere 4 a.Consequently, the potassium, cesium, manganese, and antimony are reactedon the internal surface of the upper hemisphere 4 a to form thephotocathode 11.

The partition wall 26 isolates the alkali sources 27, 27 from theelectron detection section 10. This prevents the potassium and cesiumfrom being adhered to the insulating tube 9 to thereby prevent adecrease in work function of the surface of the insulating tube 9,resulting in prevention of a reduction in voltage resistance or adverseinfluence on the electrical field in the electron tube 1. Further, thepotassium and cesium can be prevented from being adhered to the APD 15to thereby prevent a decrease in detection efficiency of the electron.The getter absorbs the impurity within the envelope 2 and helps keep thedegree of vacuum at an appropriate level.

Thus, the photocathode 11 is formed on the entire inner surface of theupper hemisphere 4 a.

Next, the glass tube 63 is removed from the exhaust device (not shown)and the end portion 65 thereof is air-tightly sealed immediately.

The electron tube 1 is manufactured in the process described above.

Operation of the electron tube 1 will next be described.

The outer stem 6 is grounded. As a result, a ground voltage is appliedto the photocathode 11 through the connection electrode 12 andconductive thin film 13.

A voltage of, for example, 20 KV is applied to the input terminal N4 ofthe electrical circuit 90. As a result, a voltage of 20 KV is applied tothe breakdown voltage control layer 44 of the APD 15, i.e., the electronincident surface 44 a of the APD 15 through the pin 32.

A voltage of, for example, 20.3 KV is applied to the input terminal N3of the electrical circuit 90. As a result, a reverse-bias voltage of20.3 KV is applied to the APD stem 16, base 87, and conductive supportportion 89 through the pin 30.

The insulating tube 9 electrically insulates from each other theconductive support portion 89, to which a positive high voltage isapplied, and the outer stem 6 that is grounded. Accordingly, theenvelope 2 and APD 15 are electrically insulated from each other,preventing a high voltage from being exposed to the outside environment.Therefore, handling of the electron tube 1 becomes easier. Further,occurrence of discharge between the electron tube 1 and outsideenvironment can be prevented. As a result, the electron tube 1 can beused even in water.

The APD 15 is provided on the inner stem 80, which is disposed on thetip end of the insulating tube 9 that protrudes inside the envelope 2.That is, the APD 15 is electrically insulated from the envelope 2 at theposition that is distant from the envelope 2. Therefore, the electricalfield inside the envelope 2 is not disturbed. As a result, electronsemitted from the electrical surface 11 can be efficiently converged ontothe APD 15 and enter the APD 15.

If the insulating tube 9 does not protrude inside the envelope 2, a partof the envelope 2 has to be formed by an insulating material in order toinsulate the APD 15 from the envelope 2. In the embodiment of thepresent invention, however, the insulating tube 9 is disposed protrudingthe inside the envelope 2, so that it is not necessary to insulate theAPD 15 and envelope 2 from each other at a portion of the envelope 2.Therefore, the photocathode 11 can be widely formed on the inner surfaceof the envelope 2, thereby increasing light detection sensitivity.

When light enters the photocathode 11 of the electron tube 1, thephotocathode 11 emits electrons in response to the incident light.Hereinafter, trajectories L of electrons in the envelope 2 will bedescribed below in greater detail with reference to FIG. 9.

As shown in FIG. 9, the APD 15 is disposed on the glass bulb body 4 side(i.e., upper side in FIG. 9) relative to the reference point S. A pointc denotes the center of the glass bulb body 4.

In this case, concentric spherical equipotential surfaces E aregenerated by a potential difference between the envelope 2 and theelectron incident surface 44 a of the APD 15. Thus, electrons emittedfrom the photocathode 11 fly along the trajectories L in FIG. 9.Therefore, the electrons emitted from the photocathode 11 are convergedon a point P1 near the upper surface of the APD 15, which is locatedslightly below the point c.

The APD 15 is disposed on the glass bulb body 4 side relative to thereference point S. More specifically, the APD 15 is disposed at thepoint P1 which is a convergent point of the electrons. Accordinglyelectrons emitted from the photocathode 11, which has substantially thehemispherical shape and which has a wide effective area, can beconverged onto a narrow area. As a result, the electrons, which areemitted from the photocathode 11 having a wide effective area, canefficiently enter the APD 15 having a small effective area, therebyincreasing detection efficiency.

Assume here, as a comparison example, that the APD 15 is disposed on thelower side relative to the reference point S in the glass bulb base 5.In this case, the equipotential surfaces E are generated as shown inFIG. 10 by a potential difference between the envelope 2 and the APD 15.Electrons are emitted from the photocathode 11 along trajectories L ofFIG. 10. As a result, the electrons from the photocathode 11 areconverged on a point P2. The electrons diffuse at the position of theAPD 15, as shown in FIG. 10. Therefore, the electrons emitted from thephotocathode 11 may not enter the APD 15 efficiently.

In the embodiment of the present invention, the APD 15 is covered by thecover 71. As a result, the incident direction of the electron is furtherrestricted to thereby further increase electron detection sensitivity ofthe APD 15.

Further, the upper end of the partition wall 26 is located on the lowerside relative to the imaginary extended curved surface I and,accordingly, does not protrude on the glass bulb body 4 side. Further,the upper end of the partition wall 26 is located on the lower siderelative to the APD 15. Therefore, the electrical field in the glassbulb body 4 can be prevented from being disturbed by the partition wall26.

In addition, the APD 15 has high-speed response, has small leak current,and can be produced with a low manufacturing cost due to a small numberof manufacturing components.

Effects of the conductive flanges 21 and 23 will next be described withreference to FIG. 11.

The upper end portion of the insulating tube 9 is connected to theconductive support portion 89, to which a positive high voltage isapplied. On the other hand, the lower end portion of the insulating tube9 is connected to the stem inner wall 61 connected to the ground. In theembodiment of the present invention, the conductive flange 21 isprovided at the connection portion between the upper end portion of theinsulating tube 9 and conductive support portion 89, and the conductiveflange 23 is provided at the connection portion between the lower endportion of the insulating tube 9 and conductive stem inner wall 61. Thisconfiguration can reduce the potential gradient in the vicinity of theconnection portions between the insulating tube 9 and conductive supportportion 89 and between the insulating tube 9 and stem inner wall 61.Therefore, this construction can prevent concentration of theequipotential surfaces and prevent the potential gradient from beingincreased. This construction can also prevent the concentric sphericalequipotential surfaces E from being distorted in the vicinity of theupper and lower portions of the insulating tube 9. Electrons emittedfrom the photocathode 11 can efficiently enter the APD 15. Light thathas entered the photocathode 11, can be detected with high sensitivity.Further, the reduction in the potential gradient reduces the electricfield intensity, thereby preventing discharge from occurring at theupper and lower end portions of the insulating tube 9. Therefore, alarge potential difference can be applied between the envelope 2 and APD15, further increasing detection sensitivity.

Further, the tip end portions 21 c and 23 d of the conductive flanges 21and 23 have thicker cross-sections than the cross-sections of otherportions thereof and have curved surfaces. Therefore, the electricalfield is prevented from concentrating on the tip ends of the conductiveflanges 21 and 23.

As described above, the potential gradient in the vicinity of the upperand lower portions of the insulating tube 9 is reduced by the conductiveflanges 21 and 23 and, thereby, the substantially concentric sphericalequipotential surfaces are formed in the electron tube 1. Thus, even ifan electron emitted from the photocathode 11 is reflected by the APD 15,this reflected electron can enter the APD 15 once again, minimizingdegradation in detection efficiency which will possibly be caused by thereflected electron. Further, the equipotential surfaces havesubstantially the concentric spherical shapes, so that the electronsemitted from any position of the photoelectrical surface 11 enter theAPD 15 at substantially the same time. Therefore, the incident time ofthe incident light on the photocathode 11 can accurately be measuredirrespective of the incident position.

If the conductive flanges 21 and 23 are not provided, as shown in FIG.12, a plurality of equipotential surfaces E concentrate on an area V inthe vicinity of the upper end portion of the insulating tube 9 and anarea W in the vicinity of the lower end portion of the insulating tube 9to generate a large potential gradient. Therefore, electrons emittedfrom the photocathode 11 are disturbed in the areas V and W to preventthe electrons from efficiently entering the APD 15, resulting in adecrease in sensitivity and an increase in noise. Further, since thereis a possibility that discharge may occur in the vicinity of the areas Vand W, a large potential difference cannot be applied between theenvelope 2 and the APD 15.

After entering the APD 15, the electrons from the photocathode 11 havelost energy in the APD 15 and, at this time, generate a large number ofelectron-hole pairs. Further, the electrons are multiplied by avalanchemultiplication. As a result, the electrons in the APD 15 are multipliedby about 10⁵ in total.

The multiplied electrons are outputted as detection signals through thepin 32. Low frequency components are then removed from the detectionsignals by the capacitor C2, and only pulse signals caused by theincident electrons are inputted to the amplifier A1. The amplifier A1amplifies the pulse signals. The pin 30 is AC-connected to the outputterminal N1 through the capacitor C1, and grounded. Therefore, theexternal circuit 100 can accurately detect the amount of the electronsthat have entered the APD 15 as a potential difference generated in theresistance R connected between the output terminals N1 and N2.

The capacitors C1 and C2 in the insulating tube 9 are located near theAPD 15. Therefore, the capacitors C1 and C2 can supply the externalcircuit 100 with low noise output signals from which direct currentcomponents have been removed, without impairing response of the signalsoutputted from the APD 15.

As described above, according to the electron tube 1 of the embodimentof the present invention, even if a ground voltage is applied to theenvelope 2 and a positive high voltage is applied to the APD 15, thevoltage applied to the connection portion between the insulating tube 9and outer stem 6 can be set to the ground voltage, preventing a highvoltage from being exposed to the outside environment. Therefore, theelectron tube 1 can easily be handled and occurrence of dischargebetween the envelope 2 and outside environment can be prevented.Further, the electron tube 1 can be used in water and can be used, forexample, in water Cerenkov experiment.

The photocathode 11 is formed on a predetermined portion of the glassbulb body 4 having a curved surface which has substantially a sphericalshape, so that the photocathode 11 can widely be formed. The APD 15 isprovided on the glass bulb body 4 side relative to the reference point Sin the glass bulb base 5, allowing the electrons emitted from thephotocathode 11 having a wide effective area to be converged on the APD15 having a small effective area. As a result, the generated electronsare converged on and enter the semiconductor device 15 in an efficientmanner, thereby increasing electron detection sensitivity. Further,since the APD 15 has a small effective area, the APD 15 has high-speedresponse, small leak current, and can be produced with a lowmanufacturing cost.

The alkali source 27 and insulating tube 9 are isolated from each otherby the partition wall 26. Therefore, when the alkali source 27 generatesalkali metal vapor to form the photocathode 11 on the predeterminedportion of the envelope 2, the alkali metal can be prevented from beingdeposited on the insulating tube 9. By preventing the alkali metal frombeing adhered to the insulating tube 9, this construction can preventthe adhered alkali metal from reducing the voltage resistance and fromhaving a bad influence to electrical field in the vicinity of theinsulating tube 9. Therefore, electrons can efficiently be detected.

The manganese bead 17 and antimony bead 19 are surrounded by the tubularouter wall 74. Therefore, when the photocathode 11 is formed, the outerwall 74 can prevent the metal vapor from being adhered to portions otherthan the upper hemisphere 4 a of the envelope 2 with a simple structureand minimal size. By limiting the photocathode 11 to a minimallyrequired area (upper hemisphere 4 a), the electrons are not emitted fromthe portions other than the effective area of the envelope 2, reducingcontribution of a dark current to the signal.

The APD 15 is surrounded by the cover 71 and tubular inner wall 72.Since the inner wall 72 prevents the metal vapor of manganese orantimony from being adhered to the APD 15, the characteristics of theAPD 15 is prevented from degrading with a simple structure and minimalsize. Further, limitation on the incident direction of thephotoelectrons further increases detection sensitivity.

The manganese bead 17 and antimony bead 19 are disposed in the vicinityoutside the APD 15, so that the metal vapor of manganese or antimonydiffuses all over the upper hemisphere 4 a. Therefore, the photocathode11 can widely be formed on the entire upper hemisphere 4 a.

When the signal from APD 15 is detected, the capacitors C1 and C2 in theinsulating tube 9 which are located near the APD 15 remove directcurrent components, so that response is not affected. Further, theelectrical circuit 90 is encapsulated inside the insulating tube 9 withthe filling material 94, so that humidity resistance is increased andthereby the electron tube 1 can easily be used in water. This preventsrespective components of the electrical circuit 90 except for theterminals N1 to N4 from directly being touched by hands, increasingsafety.

<First Modification>

As shown in FIG. 13, the vertical cross-section of the glass bulb body 4including the axis Z may be substantially a circular shape. In thiscase, the diameter of the glass bulb body 4 perpendicular to the axis Zis substantially equal to the diameter thereof parallel to the axis Z.

Also in this case, the APD 15 may be disposed on the glass bulb body 4side (upper side in FIG. 13) relative to the reference point S at whichthe imaginary extended curved surface I of the lower hemisphere 4 b ofthe glass bulb body 4 crosses the axis Z in the glass bulb base 5. Thepoint c denotes the center of the glass bulb body 4.

Equipotential surfaces E are generated by a potential difference betweenthe envelope 2 and the APD 15 and, accordingly, the electrons from thephotocathode 11 fly along the trajectories L. Therefore, the electronsare converged on a point P3 in the vicinity of the upper surface of theAPD 15, which is located slightly below the point C.

By disposing the APD 15 on the glass bulb body 4 side relative to thereference point S as described above, the electrons emitted from thephotocathode 11 can efficiently enter the APD 15, thereby increasingdetection efficiency.

As a comparison example, a case where the APD 15 is disposed on thelower side relative to the reference point S is shown in FIG. 14. Inthis case, the equipotential surfaces E are generated as shown in FIG.14 by a potential difference between the envelope 2 and the APD 15.Accordingly, electrons are emitted from the photocathode 11 alongtrajectories L of FIG. 14. As a result, electrons from the photocathode11 are converged on a point P4. The electrons diffuse at the position ofthe APD 15, as shown in FIG. 14. Therefore, the electrons emitted fromthe photocathode 11 may not enter the APD 15 efficiently.

<Second Modification>

In the above embodiment, the leading end 21 c of the conductive flange21 has a rounded shape having a greater thickness than that of theflange body 21 b. Alternatively, however, the configuration of theleading end 21 c of the conductive flange 21 may be obtained by rollingup the outer periphery of the flange body 21 b, as shown in FIG. 15.

Similarly, the configuration of the leading end 23 d of the conductiveflange 23 may be obtained by rolling up the outer periphery 23 d of therising portion 23 c.

<Third Modification>

As described with reference to FIG. 3, in the above embodiment, the cap73 of the shield portion 70 has the inner wall 72, ceiling 76, and outerwall 74. Alternatively, however, the inner wall 72 and ceiling 76 may beremoved from the cap 73, as shown in FIG. 16. In this case, the cap 73is constituted by only the outer wall 74.

Also in this case, the manganese beads 17 and antimony beads 19 aredisposed at the portions on the upper side (i.e., the upper hemisphere 4a side) relative to the base 87 and between outer wall 71 a of the cover71 and imaginary extended curved surface M of the outer periphery 87 bof the base 87, as in the above embodiment which has been described withreference to FIG. 1. Therefore, the base 87 and outer wall 74 preventsthe manganese vapor or antimony vapor from being adhered to the internalsurface of the glass bulb base 5, the outer stem 6, or lower hemisphere4 b. Further, the cover 71 prevents the manganese vapor or antimonyvapor from being adhered to the APD 15.

Further, as shown in FIG. 17, the entire cap 73 may be removed from theshield portion 70. In this case, the shield portion 70 is constituted byonly the cover 71. Also in this case, the manganese beads 17 andantimony beads 19 are disposed at the portions on the upper side (i.e.,the upper hemisphere 4 a side) relative to the base 87 and between outerwall 71 a of the cover 71 and imaginary extended curved surface M of theouter periphery 87 b of the base 87, as in the above embodiment whichhas been described with reference to FIG. 1. Therefore, the base 87prevents the manganese vapor or antimony vapor from being adhered to theinternal surface of the outer stem 6, or glass bulb base 5. Further, thecover 71 prevents the manganese vapor or antimony vapor from beingadhered to the APD 15.

Although not shown, the cap 71 only needs to have the outer wall 71 a.That is, the cap 71 need not always include the ceiling 71 b. This isbecause the outer wall 71 a can prevent the manganese vapor and antimonyvapor from being adhered to the APD 15.

<Other Modifications>

In the above embodiment, the stem bottom 60, stem is outer wall 62, andstem inner wall 61 that constitute the outer stem 6 are formed fromKovar metal. Alternatively, however, the stem bottom 60, stem outer wall62, and stem inner wall 61 may be formed from conductive material otherthan the Kovar metal.

Further, only the stem inner wall 61 to be connected to the insulatingtube 9 needs to be formed from a conductive material. The stem bottom 60and stem outer wall 62 may be formed from an insulating material.Further, only a part of the stem inner wall 61 that is connected to theinsulating tube 9 may be formed from a conductive material.

In the above embodiment, the base 87 and APD stem 16 that constitute theinner stem 80 are formed from a conductive material. Alternatively,however, the base 87 and APD stem 16 may be formed from an insulatingmaterial. At least the connection portion with the pin 30 in the APDstem 16 needs to be formed from a conductive material.

The photocathode 11 may be formed not on the entire surface of the upperhemisphere 4 a, but on a part (for example, an area around the axis Z)of the surface of the upper hemisphere 4 a. In this case, the conductivethin film 13 is formed on a part of the glass bulb body 4 at which thephotocathode 11 has not been formed, and electrical continuity isestablished between the photoelectrical surface 11 and conductive thinfilm 13.

The partition wall 26 need not always be formed from a conductivematerial. Any material can be used to form the partition wall 26 as longas the material can prevent the vapor from the alkali sources 27 and 27from being deposited onto the electron detection section 10 and does notdisturb the electrical field in the electron tube 1.

The numbers and positions of manganese beads 17 and antimony beads 19are not limited to those described above. Different numbers of manganesebeads 17 and antimony beads 19 may be provided at different positions onthe base 87.

In the above embodiment, the inner stem 80 includes the APD stem 16 andthe base 87 and the APD stem 16 is fixed to the base 87 so as to coverthe through-hole 87 a formed in the base 87. Alternatively, however, thebase 87 may be formed into substantially a circular shape and the innerstem 80 may be constituted by only the circular-shaped base 87. In thiscase, the APD 15 is disposed at substantially the center of the base 87.

Each of the conductive flanges 21 and 23 has a plate-like shape thatcircumferentially extends from the axis Z of the cylindrical electrondetection section 10 to the cylindrical glass bulb base 5 on the planeperpendicular to the axis Z. However, the configuration of theconductive flanges 21 and 23 is not limited to this. The conductiveflanges 21 and 23 only need to protrude from the upper and lower endportions of the insulating tube 9 in the direction away from the axis Zto thereby reduce concentration of the equipotential surfaces in thevicinity of the upper and lower end portions of the insulating tube 9.Further, the outer peripheries of the conductive flanges 21 and 23 neednot always be rounded.

When there is no possibility that the equipotential surfaces concentrateon the upper end portion of the insulating tube 9, the conductive flange21 need not be provided. Similarly, when there is no possibility thatthe equipotential surfaces concentrate on the lower end portion of theinsulating tube 9, the conductive flange 23 need not be provided.

If no disadvantage is found, a negative voltage may be applied to theenvelope 2 and a ground voltage may be applied to the APD 15.

The exhaust pipe 7 may be provided not at a portion between theinsulating tube 9 and partition wall 26 but at other portions such as aportion between the partition wall 26 and glass bulb base 5.

The insulating tube 9 may be formed not into a cylindrical shape butinto a square tubular shape.

Any type of an electron-bombarded semiconductor device may be adopted inplace of the APD 15.

The APD 15 may be provided on the lower side relative to the referencepoint S as far as detection of the electron can satisfactorily beperformed.

The alkali sources 27 and 27 are disposed facing each other with respectto the insulating tube 9. Alternatively, however, the alkali sources 27and 27 may adjacently be disposed. By adjacently disposing the alkalisources 27 and 27, work simplification can be achieved. For example, thealkali sources 27 and 27 can be heated by only one electromagnet.

Although the amplifier A1 is provided within the insulating tube 9 inorder to detect signals more clearly in the above embodiment, theamplifier A1 need not always be provided. In this case, the capacitor C1is directly connected to the output terminal N2.

While the preferred embodiment of the electron tube according to thepresent invention has been described with reference to the drawings, thepresent invention is not limited to the above embodiment. It will beapparent to those skilled in the art that various changes andmodifications are possible without deviating from the broad principlesand spirit of the present invention which shall be limited solely by thescope of the claims appended hereto.

The insulating tube 9 need not always be provided. In this case, theconductive support portion 89 of the electron detection section headportion 8 may be air-tightly connected to stem inner wall 61.

The capacitors C1, C2, and amplifier A1 of the electrical circuit 90 maybe provided not inside the insulating tube 9 but outside the electrontube 1.

The alkali sources 27 and 27 need not always be provided inside theelectron tube 1. Alternatively, an inlet of the alkali metal vapor isformed in the envelope 2 and the alkali metal vapor is introduced fromthe outside through the inlet to thereby form the photocathode 11. Inthis case, the partition wall 26 need not be provided.

INDUSTRIAL APPLICABILITY

The electron tube according to the present invention, which can be usedin various photodetection techniques, is in particular effective insingle photon detection in water, such as the water Cerenkov experiment.

1. An electron tube comprising: an envelope formed with a photocathodeat a predetermined part of the internal surface thereof; a fixing platewhich is disposed in the envelope and which has a central position andan outer periphery surrounding the central position; anelectron-bombarded semiconductor device which is fixed to the centralposition of the fixing plate and which faces the photocathode; a firsttubular wall which is fixed to a position between the central positionand the outer periphery of the fixing plate, the first tubular wallsurrounding the semiconductor device and extending toward thephotocathode; an evaporation source generating metal vapor, theevaporation source being disposed inside the envelope on thephotocathode side relative to the fixing plate and being disposed at aposition between the first tubular wall and animaginary-extended-curved-surface of the outer periphery of the fixingplate that extends toward the photocathode, the semiconductor devicedetecting photoelectrons emitted from the photocathode in response to anincident light thereon; and a cover which is disposed at a positionbetween the electron-bombarded semiconductor device and thephotocathode, the cover being surrounded by the first tubular wall. 2.The electron tube as claimed in claim 1, further comprising aninsulating tube having one end and another end, the another end beingconnected to the envelope and the one end protruding inside theenvelope, wherein the fixing plate and the evaporation source aredisposed on the one end of the insulating tube.
 3. The electron tube asclaimed in claim 1, wherein the envelope includes a cylindrical base;and a main body having a first main body that is curved substantially ina spherical shape and a second main body that is curved substantially ina spherical shape and that connects the first main body to the base; andwherein the semiconductor device is disposed on the main body siderelative to an intersection between an axis of the base and an imaginaryextended surface of the second main body that is located inside thebase.
 4. The electron tube as claimed in claim 2, wherein the anotherend of the insulating tube is connected to the envelope and the one endof the insulating tube protrudes inside the main body of the envelope,and wherein the fixing plate and the evaporation source are disposed onthe one end of the insulating tube.
 5. The electron tube as claimed inclaim 2, further comprising a conductive member provided on the one endof the insulating tube and protruding outside the insulating tube toreduce a field intensity in the vicinity of the one end of theinsulating tube, wherein the fixing plate includes an inner stem that isconnected to the one end of the insulating tube via a conductive member.6. The electron tube as claimed in claim 2, further comprising aconductive member provided on the another end of the insulating tube andprotruding outside the insulating tube to reduce a field intensity inthe vicinity of the another end of the insulating tube, wherein theenvelope includes an outer stem connected to the another end of theinsulating tube, at least a part of the outer stem that is connected tothe another end of the insulating tube being conductive.
 7. The electrontube as claimed in claim 1, wherein the envelope is connected to aground potential, and wherein the semiconductor device is connected to apositive potential.
 8. The electron tube as claimed in claim 1, whereinthe first tubular wall extends from one end to another end with respectto a first direction from the fixing plate to the photocathode, whereinthe cover is disposed on an upstream side of the another end of thefirst wall with respect to the first direction.
 9. The electron tube asclaimed in claim 8, further comprising a second tubular wall whichextends, along the imaginary-extended-curved-surface, from one end toanother end with respect to the first direction, wherein the evaporationsource is disposed at the same position with the another end of thesecond tubular wall with respect to the first direction.
 10. An electrontube comprising: an envelope formed with a photocathode in apredetermined part of an internal surface thereof; an electron-bombardedsemiconductor device provided inside the envelope; a first tubular wallwhich surrounds the semiconductor device; an evaporation source thatgenerates metal vapor, the evaporation source being disposed within theenvelope and outside the first tubular wall; a second tubular wall whichsurrounds the evaporation source; and a cover which is disposed at aposition between the electron-bombarded semiconductor device and thephotocathode, the cover being surrounded by the first tubular wall, thesemiconductor device detecting photoelectrons emitted from thephotocathode in response to an incident light thereon.
 11. The electrontube as claimed in claim 10, further comprising an insulating tubehaving one end and another end, the another end being connected to theenvelope and the one end protruding inside the envelope, wherein thesemiconductor device, the first tubular wall, the evaporation source,and the second tubular wall are disposed on the one end of theinsulating tube.
 12. The electron tube as claimed in claim 10, whereinthe envelope includes a cylindrical base; and a main body having a firstmain body that is curved substantially in a spherical shape and a secondmain body that is curved substantially in a spherical shape and thatconnects the first main body to the base; and wherein the semiconductordevice is disposed on the main body side relative to an intersectionbetween an axis of the base and an imaginary-extended-curved-surface ofthe second main body that is located inside the base.
 13. The electrontube as claimed in claim 11, wherein the another end of the insulatingtube is connected to the envelope and the one end of the insulating tubeprotrudes inside the main body of the envelope, and wherein thesemiconductor device is disposed on the one end of the tube.
 14. Theelectron tube as claimed in claim 11, further comprising: an inner stemconnected to the one end of the insulating tube via a conductive member;and a conductive member provided on the one end of the insulating tubeand protruding outside the insulating tube to reduce a field intensityin the vicinity of the one end of the tube, wherein the semiconductordevice is disposed on the inner stem.
 15. The electron tube as claimedin claim 11, further comprising a conductive member provided on theanother end of the insulating tube and protruding outside the insulatingtube to reduce a field intensity in the vicinity of the another end ofthe insulating tube, wherein the envelope includes an outer stemconnected to the another end of the insulating tube, at least a part ofthe outer stem that is connected to the another end of the insulatingtube being conductive.
 16. The electron tube as claimed in claim 10,wherein the first tubular wall extends from one end to another end withrespect to a first direction that is defined from the electron-bombardedsemiconductor device to the photocathode, wherein the cover is disposedon an upstream side of the another end of the first wall with respect tothe first direction.
 17. The electron tube as claimed in claim 16,wherein the second tubular wall extends from one end to another end withrespect to the first direction, wherein the evaporation source isdisposed at the same position with the another end of the second tubularwall with respect to the first direction.
 18. An electron tubecomprising: an envelope formed with a photocathode at a predeterminedpart of the internal surface thereof; a fixing plate which is disposedin the envelope and which has a central position and an outer peripherysurrounding the central position, the fixing plate holding an electrodethat is insulated from the fixing plate by an insulating material; anelectron-bombarded semiconductor device which is fixed to the centralposition of the fixing plate and which faces the photocathode; a firsttubular wall which is fixed to a position between the central positionand the outer periphery of the fixing plate, the first tubular wallsurrounding the semiconductor device and extending toward thephotocathode; an evaporation source generating metal vapor, theevaporation source being disposed inside the envelope on thephotocathode side relative to the fixing plate and being disposed at aposition between the first tubular wall and animaginary-extended-curved-surface of the outer periphery of the fixingplate that extends toward the photocathode, the semiconductor devicedetecting photoelectrons emitted from the photocathode in response to anincident light thereon, the evaporation source being connected to theelectrode; and a covering member that is located between the fixingplate and the photocathode and that covers at least a part of the fixingplate where the fixing plate holds the electrode, the covering memberexposing the evaporation source to the photocathode.
 19. An electrontube comprising: an envelope formed with a photocathode in apredetermined part of an internal surface thereof; an electron-bombardedsemiconductor device provided inside the envelope; a base which isdisposed in the envelope and which has an outer periphery, the baseholding an electrode that is insulated from the base by an insulatingmaterial a first tubular wall which surrounds the semiconductor device;an evaporation source that generates metal vapor, the evaporation sourcebeing disposed within the envelope and outside the first tubular wall,the evaporation source being connected to the electrode; a secondtubular wall which surrounds the evaporation source; and a coveringmember that is located between the base and the photocathode and thatcovers at least a part of the base where the base holds the electrode,the covering member exposing the evaporation source to the photocathode,the semiconductor device detecting photoelectrons emitted from thephotocathode in response to an incident light thereon.